You can check if the "maxversion" attribute of "product" tag is set in MXI file of the extension( For ZXP file, extract it with 7-Zip or other tools, and open MXI file with text editor ). This is the maximum version of the product that this extension can be used. For example, if this attribute is set to "7.5" in an InDesign extension, this extension can be installed in InDesign 7.5( CS5.5 ) or earlier versions, but not in InDesign 8.0( CS6 family ). As a result, you cannot install it by Extension Manager CS6.